I'm usually a little too shy to answer others' questions, but I'll give this one a shot. iCal server is pathetically under documented (thanks, Apple.), so I reckon I ought to share what I think I've figured out!
I'm not sure where you're entering your info, so bear with me if this is what you're doing:
1. Open preferences in iCal.
2. Go to the Accounts pane.
3. Click the plus in the bottom left.
4. Enter description and your user name and password (obviously user should be part of group in question)
5. Open the "Server Options" triangle and enter the URL you mention. (You're missing a closing "/" in your URL. Perhaps that matters?)
6. Click add.
7. In the left column in the main iCal window, under the "CALENDARS" section, you should now see a "WORKSHOP" section.
Now, I think that one of the biggest confusions with iCal server that I've noticed on these forums is that folks think you're only gaining access to the group's single shared calendar that's accessible through the wiki. But the really cool thing about iCal server is that it's much bigger than that - you're actually gaining access to a whole "area" in which you can create as many group calendars as you'd like. Anyone in the group who adds the group calendars through the preferences as above will have read/write access to them. You should realize, though, that none of the additional ones you make will be visible through the wiki - you can only see the one group calendar there, which frankly we found to be so error-riddled and difficult to use that we turned it off. Everyone just uses iCal rather than the web calendar.
As a side bonus, users can have their own set of calendars too. You're supposed to be able to share them (delegate access), but I can't figure that out. Just go through the preference process above to add another account with a URL ending with "/principals/users/username/". Killer.
I hope this is along the lines of what you're trying to do and somewhat helpful...